This document claims priority and contains subject matter related to Japanese Patent Application No. 11-258192 filed in the Japanese Patent Office on Sep. 13, 1999, the entire contents of which being incorporated by reference.
1. Field of the Invention
The present invention relates to a method and mold assembly for producing a molded object of plastic optical elements such as a lens, a mirror, and a prism.
2. Discussion of the Background
Metallic mold assemblies that employ an injection molding method to produce plastic optical elements such as plastic lenses with high accuracy are known. Known methods of producing a molded object 21 of a plastic optical element using a mold assembly 1 are shown in FIGS. 8A through 8C. As illustrated in FIG. 7, the molded object 21 has mirror surfaces 22 and 23 as optical surfaces, as well as non-optical surfaces 24 and 25. Referring to FIG. 8A, the mold assembly 1 includes a fixing-side metallic mold 2 providing a cavity piece 8 (i.e., a molding piece for forming a cavity) and a moving-side metallic mold 3 providing a cavity piece 9. The cavity pieces 8 and 9 have transfer surfaces 8a and 9a, respectively. A pair of porous members 26 is arranged opposing each other in the mold assembly 1. A cavity, which is filled with molten resin during molding, is defined by the cavity pieces 8 and 9 and the porous members 26. After the molten resin is loaded into the cavity, the molten resin is cooled under a controlled pressure. In the cooling process, when air is guided to the cavity via the porous members 26, the sides of the resin in the cavity corresponding to the non-optical surfaces 24 and 25 of the molded object 21 are pressed. This presses the resin in the cavity against the transfer surfaces 8a and 9a. As a result, the transfer surfaces 8a and 9a are transferred to the resin, and thereby the mirror surfaces 22 and 23 of the molded object 21 are formed.
In another mold assembly 1 illustrated in FIG. 8B, a vent hole 27 is provided at a side surface other than the transfer surfaces 8a and 9a in the cavity. Furthermore, a communication path 28 is provided to place the vent hole 27 in fluid communication with the exterior of the mold assembly 1. When the molten resin is loaded into the cavity, the communication path 28 is filled with compressed air in the cavity. Then, a differential pressure is generated between portions of the resin contacting the transfer surfaces 8a/9a and a portion of the resin contacting the vent hole 27. This forms a sink only at the portions of the resin that contact the vent hole 27. Thereby, a sink can be prevented from occurring at the mirror surfaces 22 and 23 of the molded object 21.
In still another background mold assembly 1 illustrated in FIG. 8C, a slide cavity piece 29 is provided to form a side surface of the cavity other than the transfer surfaces 8a and 9a. When the molten resin in the cavity is cooled to a temperature lower than a softening point of the resin, a gap 30 is forcibly formed between the resin and the slide cavity piece 29. This is done by sliding the slide cavity piece 29 in a direction away from the resin so as to form a sink only at a portion of the resin facing the gap 30. Thereby, a sink is prevented from occurring at the mirror surfaces 22 and 23 of the molded object 21.
In the above-described mold assembly 1, the molded object 21 of a highly accurate optical element can be obtained by forming a sink in a portion of the molded object 21 other than mirror surfaces 22 and 23 which are to be used as optical surfaces. This allows mirror surfaces 22 and 23 to be shaped by transfer surfaces 8a and 9a. This is further accomplished by reducing internal distortion of the molded object 21. However, because a portion of the resin where the sink is formed is separated from a piece of the mold assembly 1 at a temperature above the softening point of the resin, the thermal conductivity of the above-described portion of the resin where the sink is formed becomes extremely low. As a result, a significant cooling time is required to cool the resin.
Furthermore, when the sink is formed asymmetrically at a surface of the cavity other than transfer surfaces 8a and 9a as illustrated in FIGS. 8B and 8C, the temperature distribution of the resin becomes uneven. This is because the temperature of the portion of the resin where the sink is formed is relatively high and the temperature of other portions of the resin is relatively low. As a result, after the molded object 21 is removed from the mold assembly 1, the molded object 21 may deform due to differences in contraction rates. In order to prevent the deformation of the molded object 21, it is necessary to sufficiently cool the resin and to lower the temperature of the molded object 21 before removing the molded object 21 from the mold assembly 1. However, the process of cooling the resin requires a significant amount of time.
Furthermore, in the mold assembly 1 illustrated in FIG. 8C, when the slide cavity piece 29 is slid away from the resin, the molded object 21 may be deformed due to a change in the contact-force between the slide cavity piece 29 and the resin during the process of cooling the resin to a temperature lower than a softening point of the resin.
The present invention has been made in view of the above-discussed and other problems, and an object of the present invention is to address these and other problems.
The present invention provides a novel method and mold assembly for producing a molded object with a high accuracy wherein a time of cooling a resin can be reduced.
The present invention also provides a novel method and mold assembly for producing a molded object with a high accuracy wherein deformation of the molded object can be prevented.
These and other objects are achieved according to the present invention by providing and using a novel mold assembly for molding a resin. The novel mold assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity, a molding insertion member arranged to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity, and a holding insertion piece configured to hold the molding insertion member. The holding insertion piece includes a communication path that places a surface of the molding insertion member at a side opposite to the cavity in fluid communication with the exterior of the mold assembly. This is done to guide atmospheric air outside of the mold assembly toward the surface of the molding insertion member. When the resin in the cavity is cooled, the resin in the cavity shrinks and a pressure in the cavity becomes lower than a pressure of the atmospheric air guided through the communication path in the holding insertion piece. Thus, atmospheric air is drawn into the communication path toward the molding insertion member. This deflects the molding insertion member toward the cavity so as to form a sink at the other surface of the molded object and presses the at least one surface of the molded object against the at least one transfer surface of the cavity.
According to an embodiment of the present invention, a mold assembly for molding a resin, and a method of using the mold assembly, are described. The mold assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object in a predetermined shape with at least one transfer surface of the cavity, a molding insertion member arranged so as to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity, and a holding insertion piece configured to hold the molding insertion member. The mold assembly further includes another insertion piece that is provided inside the holding insertion member. This another insertion piece is configured to include at least one communication path that places a surface of the molding insertion member at a side opposite in fluid communication with a cavity side and the exterior of the mold assembly. The other insertion piece is configured to move slidably relative to the holding insertion piece in a direction away from the molding insertion member so as to lower the pressure in the cavity below the pressure of one of air, a pressurized fluid, or fluid for cooling the resin guided through the communication path. This draws the air, pressurized fluid, or fluid for cooling the resin into the communication path toward the molding insertion member. When the resin in the cavity is cooled (by moving the other insertion piece in a direction away from the molding insertion member so as to lower the pressure in the cavity below that of the pressure of air, pressure fluid, or fluid for cooling the resin) the air, pressurized fluid, or fluid for cooling the resin is drawn into the communication path toward the molding insertion member such that the molding insertion member is deflected toward the cavity. This forms a sink at the other surface of the molded object and brings the at least one surface of the molded object into tight contact with the at least one transfer surface of the cavity.
According to another embodiment of the present invention, another mold assembly for producing a molded object of resin, and a method for using the mold assembly, are described. The mold assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object in a predetermined shape with at least one transfer surface of the cavity, and a molding insertion member arranged so as to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity. The molding insertion member has a property of restoring to an original shape, and includes a convex portion protruding toward the cavity. When the resin is filled in the cavity, the molding insertion member is deformed and pressed by the filled-in resin such that the convex portion of the molding insertion member is substantially flat. As the resin in the cavity shrinks in a process of cooling the resin, the molding insertion member restores the convex portion so as to form a sink at the other surface of the molded object. This brings the at least one surface of the molded object into tight contact with the at least one transfer surface of the cavity.
According to yet another embodiment of the present invention, a mold assembly for producing a molded object from resin, and a method of using such a mold assembly, are described. The assembly includes a cavity configured to be filled with resin so as to form at least one surface of the molded object in a predetermined shape with at least one transfer surface of the cavity, and a molding insertion member arranged so as to form another surface of the molded object other than the surface formed in the predetermined shape by the at least one transfer surface of the cavity. The molding insertion member has a property of restoring to an original shape. The mold assembly further includes a holding insertion piece configured to hold the molding insertion member. The holding insertion piece includes a concave portion having a predetermined size at a surface thereof that contacts the molding insertion member. When the resin is loaded into the cavity, the molding insertion member is deformed and pressed by the loaded resin, according to a shape of the concave portion. As the resin in the cavity shrinks during cooling of the resin, the molding insertion member is restored so as to press the resin in the cavity to bring the at least one surface of the molded object into tight contact with the at least one transfer surface of the cavity.
One embodiment of a method for using an above-described mold assembly starts with filling a cavity of a mold assembly with resin to form at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity. The resin is then cooled in the cavity to solidify the resin, and a fluid is drawn through a communication path that places a surface of a molding insertion member at a side opposite to the cavity in fluid communication with an exterior of the mold assembly when the resin in the cavity is cooled, the resin in the cavity shrinks, and a pressure in the cavity becomes lower than a pressure of the fluid, said drawn fluid deflecting the molding insertion member toward the cavity to form a sink at the other surface of the molded object and to press the at least one surface of the molded object against the at least one transfer surface of the cavity.
Another embodiment of a method for using an above-described mold assembly again starts with filling a cavity of a mold assembly with resin to form at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity. Then the resin filled in the cavity is cooled to solidify the resin. Next, one of air, a pressurized fluid, and a cooling fluid is drawn into the mold assembly, the drawing done in a direction toward a molding insertion member that forms another surface of the molded object and through at least one communication path that places a surface of the molding insertion member at a side opposite to the cavity in fluid communication with an exterior of the mold assembly. The drawing includes moving another insertion piece in a direction away from the molding insertion member such that the molding insertion member is deflected toward the cavity, forms a sink at the other surface of the molded object, and presses the at least one surface of the molded object against the at least one transfer surface of the cavity.
Another embodiment of a method for using an above-described mold assembly again starts with filling a cavity of a mold assembly with resin. This filling step includes forming at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity and deforming a molding insertion member such that a convex portion of the molding insertion member is substantially flat, the molding insertion member forming another surface of the molded object and, prior to said deforming step, including the convex portion protruding toward the cavity. After filling, the convex portion of the molding insertion member restores so as to form a sink at the other surface of the molded object. This restoring step includes cooling the resin filled in the cavity to solidify and shrink the resin, and pressing the at least one surface of the molded object against the at least one transfer surface of the cavity.
Another embodiment of a method for using an above-described mold assembly again starts with filling a cavity of a mold assembly with resin. This filling step includes forming at least one surface of the molded object into a predetermined shape with at least one transfer surface of the cavity, and deforming a molding insertion member into a shape of a concave portion of a holding insertion piece that holds the molding insertion member, the molding insertion member forming another surface of the molded object. Thereafter, the molding insertion member is restored. This restoring step includes cooling the resin filled in the cavity to solidify and shrink the resin, and pressing the resin in the cavity using the restored molding insertion member such that the at least one surface of the molded object is pressed against the at least one transfer surface of the cavity.